Hearable devices or “hearables” (also known as “smart headphones,” “smart earphones,” or “smart earpieces”) are becoming increasingly popular. Such devices, which are designed to be worn over the ear or in the ear, have been used for multiple purposes, including wireless transmission and fitness tracking. As shown in FIG. 1A, the hardware architecture of a hearable typically includes a loudspeaker to reproduce sound to a user's ear; a microphone to sense the user's voice and/or ambient sound; and signal processing circuitry to communicate with another device (e.g., a smartphone). A hearable may also include one or more sensors: for example, to track heart rate, to track physical activity (e.g., body motion), or to detect proximity.